Nature has created a variety of biomass materials with various characteristics. However, all biomass materials are generally comprised of cellulose, hemicellulose and lignin. In particular, the category of biomass generally known as “wood” is comprised of approximately ⅓ by weight cellulose, approximately ⅓ by weight hemicellulose and approximately ⅓ by weight lignin. Because of the relatively high cost of fossil fuels and a desire to use more renewable resources, there is great interest in using biomass as a fuel. In addition, the use of wood waste from construction and demolition activities (C&D materials) in the preparation of a fuel product reduces the amount of material that is deposited in landfills and can comprise a valuable use of an otherwise useless material.
“Green” or unprocessed biomass is low in sulfur but has a relatively high moisture content and a relatively low heating value per unit of weight, when compared to coal, for example. Consequently, in order to create an efficient biomass-to-energy conversion, it is known to dry biomass materials and/or to torrefy biomass materials for subsequent densification. Drying of biomass materials will generally reduce the moisture content from 30-50% by weight in the green state (15-25% for C&D materials) to about 5-12% by weight, and it will increase the heating value per unit of weight because of the loss of moisture weight. However, drying will not impart significant chemical changes to the biomass materials. Further heating under controlled conditions results in torrefaction, which may be generally described as a pyrolysis or thermal degradation process carried out on wood or other biomass materials at temperatures typically less than about 600° F. in a low-oxygen atmosphere. During torrefaction, the hemicellulose in the biomass is broken down into volatile organic compounds (“VOCs”) such as methanol, formaldehyde and acetaldehyde, and at least a portion of these volatile organic compounds are released by evaporation. The removal of hemicellulose increases the energy density or heating value per unit of weight of the biomass; however, full torrefaction without recovery of the evaporated volatile organic compounds results in the loss of the energy stored therein.
A pelletized form of dried or torrefied wood or biomass is highly desirable in that it would allow easy and relatively clean handling and more efficient shipment, especially ocean freight shipment, of a biomass-based fuel. It is known to make fuel pellets from dried wood, and many attempts have been made to form fuel pellets of torrefied wood and biomass. However, the inventors have found that blends of dried and torrefied biomass materials can be pelletized to produce stable pellets of high durability. Furthermore, the inventors have devised a process that employs parallel processing of dried and torrefied biomass in such a way as to increase the energy efficiency of the process.